1. Field of the Invention
The present invention relates to a method of fabricating a compound semiconductor device and an optical semiconductor device and, more particularly, a method of fabricating a compound semiconductor device including steps of selectively growing compound semiconductor layers in predetermined regions on a growth substrate via masks by chemical vapor deposition such as metal organic vapor phase epitaxy (MOVPE).
2. Description of the Related Art
In recent years, an optical fiber has reached the stage that, because of the progress of optical communication technology and the increase in process information, it is planned to build the optical fiber in respective homes.
If the optical fibers are built in branch line systems for respective homes, it is requested that semiconductor lasers must be used at a considerably low cost in contrast to semiconductor lasers used in the trunk line systems.
Conventionally, since optical couplings between the semiconductor lasers and the optical fibers cost enormously, the fabricating methods of semiconductor integrated circuit device, by which the waveguide for converting a spot shape of a laser beam emitted from the semiconductor laser can be formed by a single crystal growth process, have been developed.
In the past, such phenomenon has been known that, when crystals are grown on the substrate having selective growth masks thereon by chemical vapor deposition such as metal organic vapor phase epitaxy, a crystal growth rate in an area near the selective growth mask is increased in comparison with the crystal growth rate in an area far from the selective growth mask.
A crystal growth having different growth rates in a plurality of areas respectively using such selective growth masks is referred to as "selective growth" hereinafter. A compound semiconductor optical device employing the selective growth is set forth in, for the first example, Tabuchi et al., "Optical Coupling Characteristics of the Semiconductor Laser with a Spot Size Converter", The Institute of Electronic Information Communication in Japan, National Autumn Meeting, 1993, Lecture Number C-182 and, for the second example, Kobayashi et al., [IEEE photon. Tech Lett., Vol. 6, pp. 1080-1081, 1994.
A compound semiconductor light emitting device with a beam conversion waveguide in which compound semiconductor light emitting devices and compound semiconductor waveguides are monolithically integrated by use of such phenomenon can be fabricated.
As set forth above, if a film thickness of a crystal layer which is selectively grown by using the selective growth mask can be controlled, the margin of device design can be enlarged in case different semiconductor devices must be formed simultaneously on the same substrate.
Next, taking as an example the case a compound semiconductor light emitting device with a beam conversion waveguide in which a compound semiconductor light emitting device and a compound semiconductor waveguide are integrated is manufactured, conventional selective growth method using the selective growth mask will be explained.
FIG. 1 is a plan view showing a substrate and selective growth masks in case a selective growth method is performed by the metal organic chemical vapor deposition. The inventors of the present invention have tried to perform the selective growth of crystal on a substrate on which selective growth masks are arranged, as shown in FIG. 1.
In FIG. 1, a reference 11 denotes a growth substrate. References rectangular M.sub.11, M.sub.12, M.sub.13, M.sub.14, M.sub.21, M.sub.22, M.sub.23 and M.sub.24 denote respectively a selective growth mask formed on the substrate. A reference 13 denotes a narrow region between the selective growth masks, which is called as a stripe portion. A reference 14 denotes a wide region between the selective growth masks, which is called as an opening portion.
In the selective growth method by means of the metal organic chemical vapor deposition using the selective masks, for example, the selective growth masks M.sub.11, M.sub.12, M.sub.13, M.sub.14, each having a length L of 600 .mu.m and a width W of 240-280 .mu.m and formed of a dielectric film such as SiO.sub.2 are aligned at a distance W.sub.1 of 10-60 .mu.m in the longitudinal direction (y direction) on the substrate 11 formed of InP etc. Similarly, the selective growth masks M.sub.21, M.sub.22, M.sub.23 and M.sub.24 are aligned at a distance L.sub.1 of 1200 .mu.m from the selective growth masks M.sub.11, M.sub.12, M.sub.13, M.sub.14 in the lateral direction (x direction) on the substrate 11.
Then, under uniform growth conditions, InP crystal layer is grown by the metal organic chemical vapor deposition on an entire surface of the growth substrate 11 on which the selective growth masks M.sub.11, M.sub.12, M.sub.13, M.sub.14, M.sub.21, M.sub.22, M.sub.23, M.sub.24 are formed as above. Thereby, for instance, there is provided a film thickness distribution having thickness differences along an x line (imaginary line) which extends from an intermediate portion of the stripe portion 13 to an intermediate portion of the other stripe portion 13 via the opening portion 14.
FIG. 2 shows a relation between a growth rate and a distance from the center portion O of the selective growth masks when pressure in the growth atmosphere is set to be 100 Torr.
FIG. 2 shows the growth rate in the longitudinal direction (x direction) of the stripe portion 13 when the InP crystal layer is grown on a (001) face of the InP substrate at a pressure of 100 Torr in the growth atmosphere, on the assumption that the width W and the length L of the selective growth masks M.sub.12, M.sub.13, M.sub.22, M.sub.23 are formed as 240 .mu.m and 600 .mu.m, respectively, that the width of the stripe portion 13 is formed as 60 .mu.m, and that the width of the opening portion 14 is formed as 1200 .mu.m.
In FIG. 2, the ordinate indicates normalized growth rate based on the growth rate of the InP crystal layer which is grown on the opening portion 14 surrounded by the selective growth masks M.sub.12, M.sub.13, M.sub.22, M.sub.23 while the abscissa indicates position from an origin O which is a center point of the stripe portion 13. In FIG. 2, a broken line on the position 300 .mu.m shows the edge of the selective growth masks M.sub.12, M.sub.13.
The film thickness distribution in FIG. 2 shows the result derived when the selective growth masks formed of SiO.sub.2 are formed on the (001) face orientation of the InP substrate and the InP crystal layer is formed thereon. More, substantially identical effect can be derived even when other crystal layer, for example, InGaAs crystal layer, is grown.
Like this, an active layer of the semiconductor laser can be formed by the crystal layer of the thick compound semiconductor grown in the stripe portion 13. Also, a waveguide portion of the semiconductor laser, which converts a spot shape of light beam emitted from the semiconductor laser, can be formed by the crystal layer of the thin compound semiconductor grown in the opening portion 14.
Consequently, if the active layer of the semiconductor laser consists of the quantum well, the well layer serving as the active layer is thick and the well layer becomes gradually thin in the waveguide region. As a result, laser light generated at the active layer spreads its optical beam shape gradually in the waveguide portion without absorption loss because an optical confinement effect is gradually reduced, and then is emitted from the cleavage face on the edge of the waveguide portion as the light which has small beam size and small beam spreading angle.
In order to improve beam spot shape conversion characteristic in the waveguide portion, it is important that a ratio of crystal growth rate of the stripe portion 13 to crystal growth rate of the opening portion 14, i.e., a selective growth ratio must be determined large.
As described above, an optical semiconductor device like the semiconductor laser in which the compound semiconductor light emitting device and the compound semiconductor waveguide are monolithically integrated can be fabricated according to the selective growth method employing the conventional selective growth masks.
However, in the prior art, it has not been reported or disclosed that distribution of growth rate in respective layers constituting the compound semiconductor light emitting device and the compound semiconductor waveguide should be freely changed in every region of plural regions by desired amount. Therefore, for example, if the selective growth ratio of the crystal in the light emitting portion is intended to be increased by narrowing the width of the stripe portion 13, the selective growth ratio of the cladding layer on the active layer is also increased correspondingly.
For instance, in general the thickness of about 1 .mu.m is required for the cladding layer in the waveguide portion. If the selective growth ratio required for the well layer is decided as "5" to effect beam conversion and the cladding layer is formed to have a thickness of 1 .mu.m in the waveguide portion under the same condition, the cladding layer of the semiconductor laser has been grown to have the thickness of 5 .mu.m.
This causes resistance of the device to increase since the film thickness has been grown in excess of required thickness as the cladding layer in the semiconductor laser. In this case, unevenness such as about 5 .mu.m is caused on the surface of the cladding layer, which makes the process difficult after crystal growth.
As the simplest countermeasure to overcome the above drawback, there is a method where the active layer of the semiconductor laser is formed by the first crystal growth, then the selective growth masks are removed, and then the cladding layer having a uniform thickness is grown on an entire surface by the second crystal growth. However, in this case, the crystal growth which must be conducted essentially only at a time is required two times, thus decreasing yield in manufacturing.
As discussed above, the technology which permits the selective growth ratio to be controlled during growing the crystal is desired. Such desire is not limited to the compound semiconductor light emitting device with the beam conversion waveguide. The desire is common to the case where different semiconductor devices should be formed simultaneously on the same compound semiconductor crystal.